So I made a compilation of satisfying substrates, and one of them I gave a challenge with, but I still put them in creations forum.viewtopic.php?f=3&t=5852
The substrate you care about is Ocean.
Rules:
1 cell inserted.
All cell types allowed.
Goal: 20+ cells
You may not remove the ocean.
All parts of your organism must touch the ocean.
Making an organism live submerged inside the ocean is bonus-point worthy.